The decision had already been made, well before the surprising news Jason Pierre-Paul required surgery to quiet down a herniated disc in his lower back. The plan was for Mathias Kiwanuka to move from linebacker back to his familiar and natural defensive end spot, a move that now takes on greater urgency with Pierre-Paul on the mend.

“The spotlight’s on you,’’ Kiwanuka said yesterday, referring to himself. “It’s an opportunity to shine.’’

The Giants do not want to consider life without JPP, and there’s hope he can return in time for the Sept. 8 regular-season opener in Dallas. That’s no sure thing, though, and coach Tom Coughlin said planning on having Pierre-Paul back for the first game is “an aggressive position to take.’’

No one in the organization views Pierre-Paul’s absence as a long-term situation, but until he returns, the Giants have a ready-made replacement capable of at least holding down the right defensive end spot. Kiwanuka, a first-round pick in 2006, was a defensive lineman until 2007, when a logjam at end prompted the move to linebacker. When Osi Umenyiora missed the entire 2008 season following knee surgery, Kiwanuka switched back to the line and had a career-high eight sacks. He also played defensive end in 2010 before a neck injury cut his season short.

“I’m ready to go,’’ Kiwanuka said. “It’s easy for me to slide back into that position because that’s what I’ve been doing.’’

There is no lack of confidence from Kiwanuka, who said “there’s no doubt in my mind’’ he can pick up where Pierre-Paul left off.

“That’s the position I came into this league playing and I’m looking forward to the opportunity,” he said. “Obviously we’d love to have him back and I’m sure he’ll be back ready to go at a certain point. We’re not going to rush him back for any reason.’’

Pierre-Paul remains in Los Angeles following Tuesday’s microdiscectomy, with an expected recovery time of 12 weeks. He did not miss a game in 2012, but his sack production dropped from 16.5 in 2011 to 6.5. At times he needed epidural injections to sooth his back and allow him to stay on the field.

“Anybody that has had a back [problem] here knows that that is a very difficult thing to deal with,’’ Coughlin said. “Nothing feels good. Your whole body suffers under that circumstance. Sometimes it is longer to recover than some other injuries.’’

Kiwanuka said it was readily apparent last season Pierre-Paul was not at full strength.

“It was obvious he was in pain, he was hurting but he was trying to push through it,’’ Kiwanuka said. “That’s the kind of player he is, but at a certain point you’re hurting yourself and you’re hurting your team. I think he realized that and decided to have surgery, which I think is the best thing for him at this point in his career.’’

The Giants won a Super Bowl with Kiwanuka starting at strong-side linebacker and, in pass-rush situations, Kiwanuka usually shifted up to the line. No one ever viewed him as a more talented linebacker than defensive end and now he gets to go back to his roots, suddenly thrust into a starting role.

“I know he’s excited to be back with his hand in the dirt and it’s really going to help us,’’ Justin Tuck said. “I think he’s going to be kind of like a sleeper at the beginning of the year because a lot of people tend to forget that he’s a pretty good defensive end.’’

The Giants must sort through several youngsters to find a third end to fill out the rotation. Adrian Tracy is next in line, followed by Adewale Ojomo and rookie third-round pick Damontre Moore. Defensive line coach Robert Nunn said Tracy “is further along than he’s been at any point in his career,’’ and added Ojomo “has got to get more consistent’’ and Moore has “flashed some things but he’s got a long way to go.’’